RDFVarBindingFilter is a function type that tests to see if
a query binding satisfies some criterion, and is used to
create a variable binding modifier that simply filers
given variable bindings.

Queries often want to apply some kind of filter or condition
to the variable bindings that are processed. In inference rules,
it sometimes seems desirable to stipulate additional conditions on
the things that are matched.

This function type is used to perform such tests.
A number of simple implementations are included.